1. Field of the Invention
The present invention is generally related to communications systems. In particular, the present invention relates to Intermediate Frequency (IF) or Radio Frequency (RF) modulators and demodulators.
2. Description of the Related Art
A component frequently found in a radio frequency (RF) transmitter/receiver (transceiver) is a quadrature modulator (QM) and/or quadrature demodulator (QD). A QM, which is also referred to as a quadrature multiplexer, modulates an intermediate frequency (IF) or an RF signal with the contents of two baseband signals. The baseband signals can include time-varying amplitude, phase, and/or frequency information. A QD demodulates, detects, or demultiplexes received IF or RF signals to recover the baseband signals.
The two baseband signals are referred to as a real or in-phase signal (often denoted by ‘I’), and an imaginary or quadrature-phase signal (often denoted by ‘Q’), and can also be described as one signal with an in-phase portion and a quadrature-phase portion. The QM mixes or multiplies each of the two baseband signals with a corresponding sinusoid or sine wave. Desirably, the two sinusoids that mix with the baseband signals are of the same amplitude, equal frequency, zero mean, i.e., no DC component, and have 90-degrees of phase separation. Conventionally, both sinusoids are derived from a common local oscillator (LO). Similarly, the QD recovers the two baseband signals by mixing a received signal with two sinusoids, again, derived from the same LO.
Conventional techniques for QM and/or QD include both digital-based and analog-based circuits. A digital-based QM or QD provides relatively accurate quadrature modulation and demodulation, but is relatively complex and expensive. Digital-based quadrature modulation/demodulation requires relatively high conversion rates from digital-to-analog converters and from analog-to-digital converters. Also, where relatively high conversion rates are desired, a digital-based QM or QD frequently requires the addition of a frequency up-converter or a frequency down-converter, respectively, thereby adding expense to the QM or QD. Further, image-rejection filters that accompany the up-converter add additional cost to a digital-based QM.
An analog-based QM or QD is desirably less expensive than a digital-based QM or QD. A typical analog-based QM directly converts to the Intermediate Frequency (IF) or to the Radio Frequency (RF) without the need for a frequency up-converter or the frequency up-converter's associated filters. Moreover, an analog-based QM or QD typically allows the use of lower-speed and less expensive digital-to-analog converters and analog-to-digital converters than a digital-based QM or QD.
Disadvantageously, conventional analog-based QM or QD circuits exhibit less accuracy in the modulation or multiplexing process than digital-based QM or QD circuits. Deviations from an ideal quadrature modulation or demodulation are generally referred to herein as “quadrature impairments.” Quadrature impairments can result from conditions such as gain imbalance, phase imbalance, and local oscillator (LO) feed-through. These conditions can be caused by component variability and by variations with respect to temperature, frequency, power, aging, time, and the like. Further, related components such as digital-to-analog converters and reconstruction filters can also contribute to quadrature impairment.
Gain imbalance occurs where the two sinusoids have unequal power, when the two baseband signals (input signals to the quadrature modulator, or output signals from the quadrature demodulator) are amplified/attenuated by different amounts by the quadrature modulation or demodulation device or by supporting hardware, e.g., filters and the like. Phase imbalance occurs in situations such as where the two sinusoids used in the quadrature modulation/demodulation process exhibit a phase relationship that deviates from 90-degrees, where there are differences in the group delay between the I and Q circuit paths, and the like.
LO feed-through occurs when power at the frequency of the LO used to generate the two sinusoids of the QM process is undesirably present in the output of the quadrature modulator. The power present at the LO frequency disadvantageously wastes valuable output power. Similarly, undesired coupling of the LO into a quadrature demodulator results in an undesirable DC offset in the recovered baseband signals.
Conventional compensation techniques for quadrature impairment are inadequate. Conventional techniques pre-distort the baseband signals to/from the quadrature modulator/demodulator to compensate for the quadrature impairment. However, conventional techniques suffer from several drawbacks, such as relatively low performance and relatively high cost.
Conventional compensation techniques are limited in compensation bandwidth and do not adequately compensate for quadrature impairment across an entire band. Some conventional techniques employ computationally intensive Fourier transform computations to extend the bandwidth of computation, but are still relatively limited in bandwidth.
Conventional compensation techniques can also require the use of injected test signals to determine coefficients or parameters used to compensate for the impairment. Such use of injected test signals is not feasible in all applications because of the interruption to the transmission of the baseband signals that occurs while the test signals are injected. Circuits and switches to inject test signals also add cost to the compensation system.
Conventional compensation techniques have also used diode detectors to detect an envelope of a quadrature modulated waveform. Diode detectors add to the cost of a system and are also sensitive to temperature, aging, and other environmental effects, and often require periodic recalibration. Further, a detector diode typically does not provide an ideal response, such as a predictable linear or logarithmic response, to an RF signal, which thereby limits the amount of compensation attainable with such diode detectors. In addition, detector diodes can vary over frequency, thereby further limiting the compensation capability of such systems.
Where a modulator includes both QM and QD, conventional compensation techniques fail to distinguish between the quadrature impairment caused by the quadrature modulator and the quadrature impairment caused by the quadrature demodulator.